In the manufacture of equipment used in the oilfield, it is often desirable to apply a coating to steel material so as to prevent corrosion of the material. Under many circumstances, these components are used in a highly corrosive hydrogen sulfide environment in which the material used for the formation of pipes and valves is easily corroded. Whenever hydrogen interacts with such components, there is the danger of hydrogen embrittlement which can cause a catastrophic failure of the system. Additionally, there are also a wide range of other corrosive chemicals which can damage such components.
This is particularly the case in many oil and gas pipelines. When such piping is used, the interior of the piping is commonly exposed to such highly corrosive items. As such, there has been a critical need so as to coat the interior of such pipes with a corrosion preventive material. Conventionally, when such pipes are used in a hydrogen sulfide environment, it is required that INCONEL (TM) material be used for the pipes. This INCONEL (TM) material is a chromium and nickel alloy. Through the use of such a chromium and nickel alloy, the pipe is resistive to hydrogen embrittlement. Unfortunately, such INCONEL (TM) material is extremely expensive. However, it is often the case that the value of the product from such hydrogen sulfide environments justifies the high cost of such INCONEL (TM) pipes.
Preferably, it would be desirable to coat or clad the interior contact surfaces of such conventional pipes with the INCONEL (TM) material. The critical contact surfaces of the pipe would be suitably coated with the chromium and nickel alloy while the remainder of the pipe would be formed of conventional steel material. This would significantly reduce the cost of such pipes. Unfortunately, prior to the present invention, no efficient process existed which would create a metallurgical bond between the chromium and nickel material and the inner surface of the pipe.
Typically, the cladding of a welding material to a surface of a workpiece has been accomplished by using submerged arc welding or electroslag welding with strip electrodes. The term "cladding" implies that a layer of material is applied by welding to the surface of a piece of metal, rather than the joining of two pieces together. Cladding is used for building up surfaces with similar materials or for applying a dissimilar layer. To increase the material disposition rate, traditional round wire filler materials are replaced with flat strips, which allow wide areas to be overlaid more efficiently. This strip welding is done using a flux to protect the arc and is simply a variation of submerged arc or electroslag welding. Strip welding using flux is easily applied to large parts where the spent flux, called slag, may be removed between welds. The necessity of slag removal between welds precludes the use of traditional strip overlay in confined areas.
Submerged arc welding produces a coalescence of metals by heating them with an arc between a bare metal electrode and the workpiece. The arc and molten metal are "submerged" in a blanket of granular fusible flux on the workpiece. Pressure is not used, and filler material is obtained from the electrode and sometimes from a supplemental source such as a welding rod or metal granules. In submerged arc welding, the arc is covered by a flux. This flux plays a main role in that (1) the stability of the arc is dependent on the flux, (2) mechanical and chemical properties of the final weld deposit can be controlled by flux, and (3) the quality of the weld may be affected by the care and handling of the flux. Submerged arc welding is a versatile production welding process capable of making welds with currents up to 2,000 amperes, AC or DC, using single or multiple wires of filler material.
In submerged arc welding, the end of a continuous bare wire electrode is inserted into a mound of flux that covers the area or joint to be welded. An arc is then initiated. A wire-feeding mechanism then begins to feed the electrode wire toward the joint at a controlled rate, and the feeder is moved manually or automatically along the weld seam. Additional flux is continually fed in front of and around the electrode and continuously distributed over the joint. Heat produced by the electric arc progressively melts some of the flux, the end of the wire, and the adjacent edges of the base metal so as to create a pool of molten metal beneath a layer of liquid slag. The melted bath near the arc is in a highly turbulent state. Gas bubbles are quickly swept to the surface of the pool. The flux floats on the molten metal and completely shields the welding zone from the atmosphere.
In all circumstances where such submerged arc welding is used, it is necessary to remove the slag from the surface of the weld. This is a problem on small inside diameter welds where the slag cannot be easily removed before the bore makes a full revolution.
It is known that the cladding of confined areas, such as the bore of a part, can be done with other welding processes which are gas shielded rather than flux shielded. Gas shielding produces no slag so welds may be used without having to remove slag. Unfortunately, this cladding is typically carried out by using a very small wire of the type normally used for joining two parts together. If such a small wire were used for the cladding of the interior of pipes, then it would take an extremely long and uneconomical time in which to complete the lining of the pipe. In the past, various patents have issued on strip welding techniques. For example, U.S. Pat. No. 3,272,961, issued on Sep. 13, 1966 to Maier, Jr. describes a method for making ribbed vapor generating tubes. This method includes the steps of disposing a tube on a horizontal axis, positioning a round wire welding electrode within the tube, striking an arc between the electrode and the tube, feeding a metal into the area of the arc, and continuously moving the electrode and the tube relative to each other whereby molten metal is deposited on the inside wall of the tube along a predetermined path.
U.S. Pat. No. 3,274,371, issued on Sep. 20, 1966 to Manz et. al., teaches a process for depositing metal onto a workpiece. This patent describes the use of an inert gas for the shielding of the arc. This is commonly known as a gas tungsten arc weld (GTAW) process. Arc welding is generally classed as either consumable or non-consumable electrode. The GTAW process uses a non-consumable tungsten electrode for the main heat source (i.e. the arc). It does not suggest the use of the filler material as the primary heat source.
U.S. Pat. No. 3,611,541, issued on Oct. 12, 1971 to W. R. Garrett, describes a tool joint rebuilding process. In this process, the outer periphery of a worn down tool joint on the end of a drill pipe is restored to a desired diameter by applying a weld bead to the outer periphery of the tool joint. The joint is rotated and the weld rod periodically translated during the application of the weld material. This is a standard submerged arc (SAW) process using round wires.
U.S. Pat. No. 4,149,060, issued on Apr. 10, 1979 to J. J. Barger, describes a method and apparatus for strip cladding into a corner while magnetically agitating a weld deposit. The clad strip is angled away from a corner in the plane of the electrode strip. The welding head can be positioned so as to clad into the comer without interference from an obstructing pole piece or from the welding head itself. This patent describes strip welding using a standard submerged arc (SAW) welding technique.
U.S. Pat. No. 4,237,362, issued on Dec. 2, 1980 to R. F. Arnoldy, describes a method of producing hard faced plate in which hard facing material is welded to the plate surface so as to cause the plate to shrink against and be supported by the cylindrical surface which effectively maintains the plate against substantial distortion during the welding. This patent describes a gasless version of the gas metal arc (GMA) technique using round wires.
U.S. Pat. No. 4,243,727, issued on Jan. 6, 1981 to Wisler et. al., describes a tool joint hardfacing in which the hardfacing is applied in a single application by rotating the drill pipe, providing an arc between a consumable steel wire and a pipe so as to create a weld puddle, and reciprocating the wire parallel to the pipe axis to create a band. This process is a gas metal arc (GMA) process using round wires.
U.S. Pat. No. 4,518,625, issued on May 21, 1985 to L. J. Westfall, describes arc metal spraying to spray liquid metal onto an array of high strength fibers that have been wound onto a large drum contained inside a controlled atmosphere chamber. The chamber is first evacuated to remove gaseous contaminants and then backfilled with a neutral gas up to atmospheric pressure. This process is an arc spray process which is a metallizing process and not a welding process.
U.S. Pat. No. 4,647,749, issued on Mar. 3, 1987 to P. Koshy, teaches an apparatus and method for weld cladding cylindrical objects. A wire-fed gas-shielded welding torch is supported adjacent to the cylindrical surface of a conduit to be coated such as that of a valve body, weld material being deposited circumferentially around the surface of the conduit. The welding torch includes an oscillating mechanism for providing linear horizontal movement of the welding torch head in the axial direction of the conduit to provide the desired weld bead configuration and characteristics. This is gas metal arc (GMA) welding.
U.S. Pat. No. 5,134,268, issued on Jul. 28, 1992 to T. D. Capitanescu describes a submerged arc welding system which is designed for placing strips of weld material onto the interior surfaces of a pipe. The device includes an elongated casing having a flux inlet and a welding wire feeding device. The flux inlet serves to deliver flux such that the welding tip is submerged in the flux during the welding. This is simply a submerged arc welding process.
It is an object of the present invention to provide a welding process which allows for the application of a corrosion resistant alloy onto the interior surface of a pipe.
It is another object of the present invention to provide a welding system in which a pipe may be internally clad in a time efficient and economically efficient manner.
It is still another object of the present invention to provide a welding system which eliminates the need for flux for the submerging of the welding arc.
It is another object of the present invention to provide a welding system which is easy to use, relatively inexpensive, and easy to manufacture.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.